Pulse-Width Modulation (PWM) normally produces a waveform with a fixed frequency and a variable duty cycle, and is widely used to control an electromechanical device. In most applications, a PWM signal is used in a control circuit to control the speed of a fan. That is, the speed of a fan is controlled by regulating the duty cycle of the PWM signal of a control instruction.
In a more complex system, most heat is dissipated by multiple fans, which according to the system difference, results in multiple PWM control instructions generated for multiple fans, and two or more fans controlled by one PWM signal in the system. Because multiple fans in the system are powered by the same power supply, when the speed of one fan changes, the fan requires a large instantaneous current, and the change of the speed of multiple fans has a great adverse impact on the power supply.
FIG. 1 shows requirements of a fan for a current when the duty cycle corresponding to the speed is regulated from 0 to 100%. It may be seen that the maximum current may reach 6.5 A after the start of the fan (possibly lasting more PWM signal periods), and that the current is only 2.6 A during the normal working after a period of time.
When there are multiple fans in the system and the fans are started simultaneously, the adverse impact on the power supply is obvious. FIG. 2 shows PWM speed regulation and requirements of two fans for a current when the duty cycle corresponding to the speed is regulated from 0 to 50% (when the duty cycle corresponding to the speed of the fans is regulated from 0 to 50%, the instantaneous current is not so large, and the data here is temporarily used for easy explanation). When the two fans are started at the same time, the maximum required current provided by a power supply module is 13 A, while during the normal working, the current is 5.2 A.
The prior art provides two solutions:
1. Increase the maximum output current capacity of a power converting module.
2. Reduce the starting current of the fans by fan manufacturers.
The first solution has the problem that a power converting module with large rated output power is required by a power supply module to meet the requirement for a short-term large current, while such large power is not necessary during the normal working of the fans, which results in design waste. The power supply with large output power is not only large in size, but also expensive, leading to a great increase in cost and is incompliance with the design idea.
The second solution requires manufacturers to reduce the starting current of the fans, which is based on the quality control of the fans; however, the extent of reduction achieved by different manufacturers is different, and some manufacturers may reduce the starting current, but the starting current is still larger than the normal working current. Moreover, the fans designed by different manufacturers are different in the starting current, which is not suitable for using the fans of the same specifications from different manufacturers in a system, directly leading to few choices of heat dissipation systems during the system design.